Composite membranes for flow batteries

ABSTRACT

A composite membrane for use in flow batteries is contemplated. The membrane comprises a hydrogel, such as poly(vinyl alcohol), applied to a polymeric microporous film substrate. This composite is interposed between two half cells of a flow battery. The resulting membrane and system, as well as corresponding methods for making the membrane and making and operating the system itself, provide unexpectedly good performance at a significant cost advantage over currently known systems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/080,080 filed on Aug. 27, 2018, which itself claimed priority toPatent Cooperation Treaty patent application PCT/US2017/19644 filed onFeb. 27, 2017 and U.S. provisional patent application 62/300,323, filedon Feb. 26, 2016, all of which are incorporated by reference herein, andthis disclosure was developed with funds received under a contract withthe United States Department of Energy (contract DAR0000352), who maypossess certain rights in this invention.

FIELD OF INVENTION

The present invention relates to the field of flow batteries and, morespecifically, to a system and apparatus involving a composite membranefor use in such batteries, as well as a method of making and using anyof the same.

BACKGROUND

A flow battery is a rechargeable battery that uses electrolytes moving(“flowing”) through an electrochemical cell to convert chemical energyfrom the electrolyte into electricity (and vice versa when charging).The active materials used in flow batteries are generally composed ofionized metal salts or redox active organic compounds dissolved in afluid, such as water or an organic solvent(s). Additional salts oracids, such as NaCl or HCl, may also be provided to the fluid so as tocreate a highly conductive electrolyte.

The electrolytes for flow batteries are typically stored in separatetanks and then pumped through individual cells of the flow battery in acontrolled manner, usually according to the charge/discharge currentapplied. Multiple cells may be employed (i.e., “stacked”), in series orin parallel, in order to create the desired electrical characteristicsfor the battery.

When discharging, each cell consists of a positive (cathode) andnegative (anode) electrode and a separating membrane. The electrodescatalyze the desired reactions. The membrane that allows the conductionof ions necessary to complete the electrical circuit, while preventingthe electrodes from coming into contact. The separator should alsoprevent any mixing of the circulating positive and negative electrolytesand minimize the movement of species produced in an electrolyte duringcharging from crossing over or intermingling with the other components(e.g., the other electrolyte). Additional mechanical and controlstructures may be employed to generate and sustain the desired flow ofelectrolyte/reactants through the cell(s).

A true flow battery has all the active chemical species flowing throughthe battery and stored in the external tanks. Reduction-oxidation(redox) flow batteries, such as vanadium or iron/chromium redoxbatteries, store electrical energy in a chemical form and subsequentlydispense the stored energy in an electrical form via a spontaneousreverse redox reaction. The discharged electrolyte can be flowed througha reactor cell with an external voltage source applied such thatelectrical energy is converted back to chemical energy.

‘Hybrid’ flow batteries have at least one of the active materialsphysically located within the cell/stack, such as the zinc metal in azinc-bromine battery or iron in an all-iron battery, while the otheractive materials are dissolved in the electrolyte. Such hybrid flowbatteries still utilize separate positive and negative electrolytes andrequire a separator or membrane.

A variety of chemistries have been employed in flow batteries. Thetypical cell voltages range from <1.0 up to ˜2.4 volts, withhydrogen-lithium bromate, iron-chrome, all-iron, vanadium-vanadiumsulfate (or mixed sulfate and chloride), sodium-bromine polysulfide andzinc-bromine serving as non-limiting examples.

In practice, flow batteries are similar to fuel cells, in that they relyupon electron transfer (rather than intercalation or diffusion).Similarly, flow batteries share characteristics with rechargeableconventional batteries, excepting that the active material of flowbatteries may be easily replenished outside of the cell (e.g., in thestorage tanks through the application of an appropriate electricalcharge). As such, flow batteries present advantages over conventionalbatteries, which possess limited discharge capacity based upon theactive material contained within the cell and, in the case of secondarybatteries, possess relatively limited cycling capabilities owing to theinherent limitations of repeated intercalation. Flow batteries alsopresent advantages in comparison to fuel cells because they do notrequire specialized catalysts and because reactants can be replenishedvia the application of electrical current, without any need forwholesale replacement of either the cell or the reactants. Nevertheless,to date, flow batteries have mostly found utility in larger, stationaryapplications and/or in combination with other power generation schemes,although a broad array of possibilities is anticipated.

Examples of flow batteries can be found in United States PatentPublication No. 2014/0227574; U.S. Provisional Patent Application No.62/239,469 filed on Oct. 9, 2015 and International Patent ApplicationNo. US15/50676 filed on Sep. 17, 2015. The entirety of the disclosure ineach of these documents may be incorporated by reference herein, and theinventors further reserve the right to attempt to establish priorityclaims to these applications to the fullest extent permitted byapplicable law.

One of the most common membrane materials in use today is a sulfonatedtetrafluoroethylene based fluoropolymer-copolymer sold by the E.I. duPont de Nemours and Company based in Wilmington, Del. under the tradename NAFION. This material possesses a Teflon skeleton with acidicsulfonic groups, resulting in cationic conductivity, resistance tochemical reactivity and moderate temperatures and comparatively goodwater and gas permeability. However, the proprietary nature of thematerial also entails a relatively high cost, and NAFION® has poorconductivity for flow batteries relying upon ions other thanproton-for-ionic charge movement (e.g., potassium, sodium, chloride,etc.).

Another approach to membranes is disclosed in United States PatentPublication No. 2013/011504. An ion exchange membrane is described forredox flow batteries. The membrane is made from a composition comprisingan ion conductive material, such as an ion conductive monomer or polymer(e.g., quaternary ammonium salts), and a water soluble support. In someinstances, non-effective ion materials selectively substituted in placeof the ion conductive polymers. The resulting membrane is described asbeing useful in flow batteries having non-aqueous electrolytes andsolvents.

Porous and microporous membranes have also received consideration foruse in flow batteries. However, these materials are extremely sensitiveto pressure differentials, insofar as their porous nature allows forliquid/fluid to pass through (as opposed to following the intendedflow-path for the electrolyte(s) in question). Consequently, the use ofporous membranes often requires additional components to monitor andadjust the pressure and flow of both electrolytes. To the extent poresmay become physically obstructed, the resulting localized disparities inporosity present further challenges even if such control components areemployed.

Given the foregoing, there is a need for a cost effective alternative toknown membrane materials. Further, membranes that do not require closemonitoring and adjustment of pressure differentials in a flow battery,especially via complex control components, would be welcome. A materialthat is more compatible with non-proton rich electrolytes and, morespecifically, a material that exhibits selectivity between the variouscharge states of iron (e.g., Fe²⁺, Fe³⁺) or vanadium (e.g., V²⁺, V³⁺,V⁴⁺, V⁵⁺, etc.) and other multi-valent (i.e., other ionic species,beyond iron and/or vanadium, exhibiting several different states ofvalence) and/or cationic species (e.g., K⁺, Na⁺, H⁺, etc.), are alsoneeded.

SUMMARY OF INVENTION

In view of the foregoing, a membrane for flow batteries is contemplated.This membrane consists of a hydrogel-forming material (e.g., poly(vinylalcohol), hereafter “PVA”) and an optional porous polymeric material.This membrane is disposed between the half-cells of a flow battery toallow for the selective flow of ions through the membrane withoutrelying on traditional ionomers. The membrane is expected to haveparticularly utility in the field of iron flow batteries, although itsselectivity has utility in a wider range of multi-valent flow batterychemistries.

In certain aspects, a flow battery comprises any combination of thefollowing features:

-   -   a positive electrolyte flowing through a positive reaction        chamber;    -   a negative electrolyte flowing through a negative reaction        chamber;    -   a non-ionomeric membrane comprising a hydrogel physically        separating the positive reaction chamber from the negative        reaction chamber;    -   wherein the membrane is ionically conductive but resistant to        hydraulic crossover;    -   wherein at least one of the positive and negative electrolytes        comprises an aqueous solution;    -   wherein at least one of the positive and negative electrolytes        comprise a multi-valent ionic species;    -   wherein the multi-valent species include at least one of:        vanadium (2⁺) ions, vanadium (3⁺) ions, vanadium (4⁺) ions,        vanadium (5⁺) ions, cuprous ions, cupric ions, ferric ions, and        ferrous ions;    -   wherein the hydrogel comprises poly(vinyl alcohol);    -   wherein the hydrogel is selected from: poly(vinyl alcohol),        poly(acrylic acid), poly(ethylene glycol), and combinations        thereof;    -   wherein the hydrogel effectively blocks the pores of a polymeric        substrate;    -   wherein the polymeric substrate is microporous;    -   wherein the polymeric substrate consists of polyethylene,        polypropylene, and combinations thereof;    -   wherein the polymeric substrate has a void volume between 30%        and 80%;    -   wherein the polymeric substrate has a void volume exceeding 80%;    -   wherein the membrane consists of hydrogel;    -   wherein the pH of the positive and negative electrolytes is less        than 3.0, more preferably less than 2.0, and most preferably        less than 1.5; and    -   wherein the hydrogel is at least one of: hydrolyzed,        crystalline, semi-crystalline, and crosslinked.

In another aspect, a method for making a membrane for flow batteries mayinclude any combination of the following features:

-   -   providing a substrate having a plurality of pores forming a void        volume;    -   impregnating the pores with a hydrogel;    -   treating the hydrogel to create a non-ionomeric but ionically        conductive membrane;    -   wherein the hydrogel is selected from: poly(vinyl alcohol),        poly(acrylic acid), poly(ethylene glycol), and combinations        thereof;    -   wherein the treating the hydrogel includes at least one of        thermal crosslinking, chemical crosslinking, photochemical        crosslinking, and hydrolyzing the hydrogel;    -   wherein the impregnating the pores comprises at least one of        film casting, roll-to-roll casting, infiltration, and dip        coating;    -   wherein the impregnating the pores results in a layer of        hydrogel deposited on at least one side of the substrate at a        thickness of 0.1 to 25 micrometers;    -   prior to the impregnating the pores, at least one of: cleaning        the pores and removing air from the pores;    -   submerging the substrate in alcohol;    -   wherein the hydrogel comprises poly(vinyl alcohol) selected to        have a purity of at least 99 wt. %; and    -   wherein at least 95 wt. % of the hydrogel is hydrolyzed.

In a further aspect, a membrane with tolerance for pH levels approaching1.0 that is selective for multi-valent ionic species includes anycombination of the following features:

-   -   a microporous, polymeric membrane;    -   a hydrogel impregnated in the membrane;    -   wherein the hydrogel is selected from: poly(vinyl alcohol),        poly(acrylic acid), poly(ethylene glycol), and combinations        thereof;    -   wherein the polymeric membrane consists of at least one of        polyethylene and polypropylene;    -   wherein the polymeric membrane is effectively free of any air        bubbles;    -   wherein the polymeric membrane has a porosity of between 30% and        80% and more preferably between 40% and 60%;    -   wherein the polymeric membrane has a thickness of between 25 and        200 micrometers prior impregnation of the hydrogel;    -   wherein the polymeric membrane has a thickness of less than 200        micrometers after impregnation of the hydrogel;

In a final aspect, a method of making and operating a flow battery mayinclude any combination of the following features:

-   -   providing or forming a composite membrane according to any        aspect disclosed herein;    -   providing first and second electrolytes, each having        multi-valent ionic species dissolved therein, on opposing sides        of the composite membrane; and    -   circulating the first and second electrolytes through separate        flow paths in response to an electric load or a charging load;    -   wherein the multi-valent ionic species include at least one:        vanadium (2⁺) ions, vanadium (3⁺) ions, vanadium (4⁺) ions,        vanadium (5⁺) ions, cuprous ions, cupric ions, ferric ions, and        ferrous ions;    -   wherein the first electrolyte comprises ferric ions and the        second electrolyte comprises ferrous ions; and    -   wherein at least one of the first electrolyte and the second        electrolyte has a pH of less than 3.0, more preferably less than        2.0, and most preferably less than 1.5.

Although the foregoing features are separately identified as distinctaspects of the invention, it will be understood that this disclosurealso contemplates combining features from one disclosed aspect abovewith any of the other aspects. To the extent one aspect may encompass amethod and a separate aspect may encompass an apparatus, the means foradapting the method to the apparatus (and vice versa) are similarlycontemplated and disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various systems, apparatuses,devices and related methods, in which like reference characters refer tolike parts throughout, and in which:

FIG. 1 is a schematic illustration of a hybrid flow battery.

FIG. 2 is a schematic representation of cross-linking within thehydrogel.

DETAILED DESCRIPTION

Reference will now be made to exemplary embodiments, examples of whichare illustrated in the accompanying drawings. It is to be understoodthat other embodiments may be utilized and structural and functionalchanges may be made. Moreover, features of the various embodiments maybe combined or altered. As such, the following description is presentedby way of illustration only and should not limit in any way the variousalternatives and modifications that may be made to the illustratedembodiments. In this disclosure, numerous specific details provide athorough understanding of the subject disclosure. It should beunderstood that aspects of this disclosure may be practiced with otherembodiments not necessarily including all aspects described herein, etc.

As used herein, the words “example” and “exemplary” means an instance,or illustration. The words “example” or “exemplary” do not indicate akey or preferred aspect or embodiment. The word “or” is intended to beinclusive rather than exclusive, unless context suggests otherwise. Asan example, the phrase “A employs B or C,” includes any inclusivepermutation (e.g., A employs B; A employs C; or A employs both B and C).As another matter, unless context suggest otherwise, the articles “a”and “an” are generally intended to mean “one or more” and the use ofplural may be exemplary rather than mandatory.

Hydrogel forming materials such as poly(vinyl alcohol), poly(acrylicacid) or poly(ethylene glycol), and blends of two or more of thesematerials, have found use in electrochemical cells. For example, U.S.Pat. No. 5,211,827 describes a conventional electrochemical cell using asolid, nonporous composite membrane formed by dispersing hydrogeluniformly through an inert matrix. In the same manner, U.S. Pat. No.4,614,575 describes the use of non-ionic aqueous gels to facilitateelectrolyte distribution while preventing gas percolation in a varietyof electrochemical systems. Generally speaking, the use of non-ionicpoly(vinyl alcohol) gel film separators in alkaline silver-zincbatteries dates back even further. In all cases, the challenges orincorporating soluble PVA membranes in a single aqueous electrolyte,along with lower cost, less complex alternatives, may have contributedto the relative dearth of further development of PVA separators.

For conventional batteries, the necessary ionic conduction through thehydrogel occurs as the liquid battery electrolyte penetrates and swellsthe hydrogel. The cross-linking is necessary to control the degree ofswelling and to prevent the hydrogel polymer from dissolving in theelectrolyte. For fuel cells, an ionomer such as poly-styrene sulfonicacid or NAFION® is usually incorporated into the separator. Conductionthen occurs when water is taken up by the hydrogel and allows the acidgroups of the ionomer to become ionized, freeing up a proton forconduction. However, NAFION® and other ionomers may take up (i.e.,absorb and hold) multi-valent cations (V²⁺, Fe²⁺, Cr³⁺, etc.) relativeto H⁺, Na⁺, and/or K⁺, so as to cause a significant loss in ionicconductivity that renders flow battery systems relying on multi-valentcations largely inefficient.

Recent literature (e.g., a review by Maiti in The Journal of PowerSources, vol. 216, pages 48-66, 2012) has suggested modification of PVAby sulfonation or incorporation of hydrophobic or other components couldprove useful for PVA-based membranes for direct methanol fuel cells,given PVA's high selectivity for water to alcohol. However, thesestructures rely on creating ionomeric properties in the membrane.

In contrast to the use of similar membranes for conventional batteriesand fuel cells, separators/membranes for flow batteries must provideionic conductivity and also prevent (or at least minimize) the transferof the dissolved ionic reactants from the positive electrolyte to thenegative electrolyte and vice-versa. That is, reactant transport and, inparticular, transport via hydraulic permeability are significantquestions unique to flow batteries which limit the use of conventionalbattery and fuel cell separators (e.g., low-cost microporous membranes)in flow batteries.

Reactant transfer can be driven by 1) the naturally occurringconcentration gradients (i.e, by diffusion), by 2) the naturallyoccurring difference in electrical potential (i.e., by migration) or by3) differences in pressure (total or osmotic) that result in the bulktransfer of liquid electrolyte across the membrane (hydraulicpermeation). In conventional batteries, only the ions of the supportingelectrolyte, such as potassium and hydroxyl ions for an alkalinebattery, are present in the electrolyte, so that reactant transfer isnot an issue. In fuel cells, the unwanted transport of fuel (hydrogen ormethanol) molecules may be an issue, but these materials are not ionicand their transport is controlled by different factors. Only in flowbatteries is conduction of some ions desired while conduction of otherions is to be minimized, and only in flow batteries is hydraulicpermeation of electrolyte an issue to be avoided.

Thus, the inventors now propose to leverage the benefits of microporousmembranes as support structures in combination with hydrogels, andparticularly crosslinking and conductive materials such as poly(vinylalcohol), as safeguards against extremes in pressure differentials inflow batteries. The resulting structures significantly reduce theshortcomings of only using microporous membranes in flow batteries,while simultaneously realizing significant cost savings in comparison toNAFION® and other ionomers/ion exchange materials.

Further, the optimal configuration of a composite separator using ahydrogel in flow batteries is unique in comparison even to thecomparable disclosures of hydrogel in electrochemical cells, as observedabove and elsewhere in the literature. The combination of hydrogel andmembrane in the circumstances that are specific to flow batteries (i.e.,hydraulic permeation, diffusion, etc.) entail factors such as the poresize and pore volume of the porous support, the composition and amountof hydrogel forming material and its degree of cross-linking, andwhether or not the hydrogel material forms a continuous film on thesurface of the support and/or partially or completely fills the pores ofthe support. Insofar as these factors are not relevant to or, to theinventors knowledge, even considered in the prior art with respect toconventional electrochemical cells, the composite and systemcontemplated herein is unique.

In addition to semi-crystalline, near-completely hydrolyzed PVA (seebelow) as a component of composite membranes for flow batteries, manyother options are envisioned. For example, PVA modified withcrosslinkable moieties (e.g., acrylates, epoxides) can be used toinfiltrate porous separator materials followed by thermal, chemical orphotochemical crosslinking. Alternatives to PVA which are also notionomers include hydrophilic polymers such as poly(ethylene oxide) orPEO, poly(N-vinyl pyrrolidone) or PVP, poly(2-hydroxyethyl methacrylate)or polyHEMA and the like, poly(acrylamide) and various derivatives andcopolymers, and poly(ethyl-2-oxazoline) and related materials, withappropriate crosslinking, either physical or chemical, to render thesematerials insoluble in water. Bio-derived hydrophilic polymers, such aschitosan, gelatin and hyroxypropyl cellulose, may also be considered.

Blends of one or more of these polymers may also be of interest. One ormore blend components may include a polyelectrolyte, such aspoly(acrylic acid) or a neutralized derivative such as poly(sodiumacrylate), and various copolymers of acrylic acid with, for example,acrylamide, in various amounts. Bio-derived polyelectrolytes, such asalginate and hyaluronic acid, may also be considered.

Multi-layer composite membranes are also envisioned, such as a layer ofcross-linked PVA sandwiched between two conventional porous separators.This would provide better mechanical stability and protect therelatively soft hydrogel material from being directly exposed to theflowing electrolyte.

The PVA embodiments of the invention should use exceedingly pure formsof the material. Ideally, the poly(vinyl acetate) precursor commonlyused to create the final alcohol in situ should be at least 90%, atleast 95% and, more preferred, at least 99% pure. A failure to utilizesufficiently pure PVA and/or precursor may lead to unwanted impuritiesand reduced performance of the resulting membrane.

The PVA embodiments include depositing the hydrogel material onto themicroporous membrane via film casting, roll-to-roll casting,infiltration, dip-coating, and other techniques. In doing so, the PVAshould fill or otherwise effectively block the pores of the microporousseparator so as to effectively prevent flow of fluids through the pores.The resulting surface coating on either/both sides of the membraneshould have a depth of about 0.1 to 25 micrometers on a single side. Insome embodiments, all of the pores are filled. In some embodiments, oneor both sides of the separator are coated. Ultimately, the applicationor deposition of hydrogel must be sufficient to avoid or substantiallyreduce the aforementioned problems associated with uncoated porousstructures.

A pore is considered to be effectively blocked if its effective diameteris reduced by approximately two orders of magnitude (e.g., a 100nanometer pore is reduced to an effective size of about 1 nanometer,etc.). However, it will be understood that the coating is effectivelycontinuous on its surface, and the effective blocking may involve anyof: the complete filling of the pore, significant coating of theinterior diameter of the pore along at least a portion of its paththrough the pore, and/or sufficient filling of the pore at either orboth of its surfaces so as to effectively block the pore. Ultimately,the efficacy and/or utility of the membrane is determined by thehydraulic permeation and ionic selectivity aspects of the membrane, andthe effective blocking of the pores is merely indicative of thesequalities.

Examples of microporous separators appropriate for use with theinvention include inert, thin polymer films between 1 to 1000micrometers (μm) in thickness made from polypropylene and/orpolyethylene, including a wide range of molecular weights. Membranesmade from or including polytetrafluoroethylene may also be used assupport. The pore size for such membranes should be between 0.01 and1.00 micrometers with a porosity or void volume between 30 to 80%, withmore preferred upper and lower ranges bounded by any two values selectedfrom: 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, and 80%porosity/void volume. The membranes may be uniaxial or biaxial innature, and the membranes may be comprised of multiple layers ofpolymeric materials. Other salient traits may include: stability inelectrolytes, exposure to active redox species, mechanical propertiesnecessary to support a free-standing membrane, surface traits that arecompatible with hydrogel penetration and coating, and wettability (bothfor cleaning/removal of air bubbles, as well as for penetration of thehydrogels). Preferred characteristics are further highlighted in theexamples below, which should be understood as encompassing certainaspects of the invention.

Surprisingly, the inventors determined that these composite membranespossessed improved wettability (in comparison to the microporousmembrane alone) and unexpectedly lower resistance (i.e., equal to orless than 0.3 ohm-cm²). Further, the inventors believe these compositesmay possess a certain level of ionic selectivity with respect to ferrousand ferric ionic species as compared to other cationic species.Additional testing may show similar utility with respect to selectivityfor vanadium, copper, and possibly other flow battery and hybrid-typesystems. In this context, ionic selectivity means that the relativelylarger, multi-valent cations may be conducted through the membrane, butat a slower rate than the smaller diameter monovalent cations. In thismanner, the composite membrane can be contrasted with NAFION® and otherionomers, in which a physical partitioning is accomplished by the uptakeof the cations into the NAFION® material itself. While not wishing tobound by any specific theory, the inventors believe their compositemembrane may be effectively nanoporous in this regard, by hindering thepassage of larger cations, and the composite membranes may be likened tonanofiltration membranes in this regard and in contrast to more porousmicrofiltration and ultrafiltration membranes (useful in capturingmolecular weight materials in excess of 10,000, such as proteins), butless porous than reverse osmosis membranes (useful in trace organics andmonovalent cations).

Finally, the composite separators exhibited sufficient ionicconductivity within the appropriate ionic species (i.e., greater than0.01 S/cm), particularly in comparison to the current industry standardionomeric membranes, like Nafion®. Ionic conductivity performance mayalso be reflected by the comparative performance of membranes with andwithout the invention, as reflected by the battery's discharge, cycling,and/or charging performance, as well as its rate capability and overallcapacity. Additionally, related measures, such as resistancemeasurements (e.g., area specific resistance), may be useful inverifying the improved ionic conductivity of the invention in comparisonto previous membranes.

Ionic conductivity may be measured by equilibrating a membrane in agiven electrolyte. Equilibrating includes immersing the membrane andallowing sufficient time for complete wetting of the membrane.Comparative (i.e., with and without membrane present) or multi-pointmeasurements (i.e., on and away from the membrane on both of its sides)may be taken to determine conductivity. Diffusion coefficients may alsobe calculated and compared.

For certain polymeric substrates, it may be advantageous to ensure allunwanted substances are first purged from the pores prior to theintroduction of hydrogel materials. This may be accomplished accordingto any number of procedures, including flushing the pores clear of anyresident oils or other residual components from original manufacturingprocesses by way of one or more solvents. In addition, it may beadvisable to pre-wet the membrane to avoid trapping air within thepores, which may lead to unwanted reactions. In either of these regards,methanol and other alcohols are particularly useful. In practice, thesubstrate is “pre-wetted” with the alcohol and then introduced into anaqueous solution so as to ensure air bubbles have been evacuated and/orthat the hydrogel/PVA may be immediately taken up within the pores.

The composite membranes contemplated herein may be particularly usefulin the ferric-ferrous chemistry and in other multi-valent ionic flowbattery chemistries (both traditional and hybrid). Additives may beemployed allowing sustained operation at pH as low as 1.0 or less in amixed sulfate/chloride electrolyte without experiencing infeasiblelevels of hydrogen evolution. Also, these electrolytes are sufficientlyconductive to forego possible ferric hydroxide precipitation. PreferredpH levels for the electrolytes is less than 3.0, less than 2.0, lessthan 1.5, and less than or equal to 1.0.

The composite hydrogels may include mixtures of PVA, poly(acrylic acid),and poly(ethylene glycol). The resulting bonding within the poroussupport may be covalent, ionic, or a combination of the two. Thehydrogel itself may also be partially cross-linked according to avariety of known methods to improve its structural stability,particularly with respect to minimized swelling and avoidance ofdissolution of the hydrogel material(s) themselves. In some aspects, 3wt. % aqueous solutions of highly hydrolyzed PVA (i.e., at least halfand, more preferably, approaching 90% of the material) are coated ontothe microporous substrates, followed by one or a series of methanoltreatments lasting several hours in order to stabilize the PVAstructure, increase crystallinity, and promote PVA-PVA interactionswhile minimizing PVA-water interactions.

Notably, flow batteries present unique challenges and opportunities, incomparison to previous suggested uses of PVA in conventional batteriesand fuel cells, because of the need for selective ionic conduction andsufficient physical segregation of the respective aqueous mixturesinherent to the opposing cells of a flow battery. These considerationsare in direct contrast to conventional batteries and fuel cells, wherepressure gradients and/or hydraulic crossover are not as significant ofconcerns. While it is true that fuel cells may experience fuelcrossover, the art of fuel cells are entirely unconcerned with ioncrossover.

As described herein, resistance to hydraulic permeation may mean athickness normalized hydraulic permeability, inclusive of the coatingthickness, (k_(w)/t_(m) in cm²/cm) of less than or equal to 9×10⁻¹² andor absolute values of less than or equal to 10⁻¹⁴ cm² (for comparison,an open and unblock space will have a large hydraulic permeation valuesignificantly greater than 1.000000). Another comparative metric forthis resistance the relative amount by which a membrane against one thathas been coated according to the aspects described herein comparesagainst an uncoated membrane, with a reduction in permeability of atleast 25 times in the coated membrane considered to be resistant.Pressure gradients between half cells may also be indicative of unwantedhydraulic crossover. Other measures, including indirect means such asabsolute or comparative full or half performance on discharge, cycling,and/or charging, as well as the rate capability and capacity of theresulting cell, may also be viable indicators of improved and acceptablehydraulic permeation resistance.

Owing to the ability for PVA and other hydrogels to crosslink, it may bepossible to rely on extremely porous substrates (approaching 80%porosity or more). In some aspects, a completely self-supported membranecan be formed entirely from hydrogel. Such self-supported hydrogelscomprise extremely pure PVA that may tolerate temperatures at or above50°-60° C. In other aspects, sufficient purity of PVA (i.e., greaterthan 95%) may result in the formation of crystalline structures in whichthe hydrolyzed PVA may self-crosslink. Extremely porous substrates maycomprise a simple mesh formed from threaded or woven screen-likematerials, expanded materials having regularly or irregularly formedapertures, and/or other planar materials having apertures formedtherein.

While the composite membrane is expected to have particular utility inmulti-valent flow batteries, and especially all iron systems, thecomposite membranes described herein also exhibit ionic selectivity formonovalent cations. For example, electrolyte conductivity even in liquidsolutions of K⁺ or NH₄ ⁺ and Cl⁻ can remain comparable. Separately,lower pH levels, in combination with possible hydrogen suppressantsbased on vanadium, further improves the performance of the compositemembrane in multi-valent systems.

The composite membranes also show no apparent degradation when cycledboth at room temperature and elevated temperature (50° C.).

The methods described in the specific examples below encompass otheraspects of the invention. Although specific equipment is disclosed,reasonably comparable implements and techniques are encompassed bycertain embodiments of the inventive method. In the same manner, theflow battery systems described herein constitute certain embodiments ofthe invention, especially to the extent the use of the compositemembrane simplifies and improves the performance and components of theoverall system.

FIG. 1 illustrates an embodiment of a flow battery system 100 suitablefor use in connection with aspects of the present invention. Flow cell100 includes two half-cells 102 and 104 separated by a separator 106.Half cells 102 and 104 include electrodes 108 and 110, respectively, incontact with an electrolyte such that an anodic reaction occurs at thesurface of one of the electrodes and a cathodic reaction occurs at theother electrode. Electrolyte flows through each of the half-cells 102and 104 as the oxidation and reduction reactions take place. In FIG. 1,the cathodic reaction takes place during discharge in half-cell 102 atelectrode 108 (which is referred to herein as the positive electrode orthe cathode), and the anodic reaction takes place during discharge inhalf-cell 104 at electrode 110 (which is referred to herein as thenegative electrode or the anode).

The electrolyte in half-cells 102 and 104 flows through the system tostorage tanks 112 and 114, respectively, and fresh/regeneratedelectrolyte flows from the tanks back into the half-cells. In FIG. 1,the electrolyte in half-cell 102 flows through pipe 116 to holding tank112, and the electrolyte in tank 112 flows to the half-cell 102 throughpipe 118. Similarly, the electrolyte in half-cell 104 can flow throughpipe 120 to holding tank 114, and electrolyte from tank 114 flowsthrough pipe 122 to half-cell 104. The systems can be configured asdesired to aid or control the flow of electrolyte through the system andmay include, for example, any suitable pumps or valve systems. In theembodiment depicted in FIG. 1, the system includes pumps 124 and 126 topump the electrolyte from tanks 112 and 114, respectively to thehalf-cells. In some embodiments, the holding tank can segregateelectrolyte that has flowed through the respective cells fromelectrolyte that has not. However, mixing discharged or partiallydischarged electrolyte can also be performed.

Electrodes 108 and 110 can be coupled to either supply electrical energyor receive electrical energy from a load or source. Other monitoring andcontrol electronics, included in the load, can control the flow ofelectrolyte through half-cells 102 and 104. A plurality of cells 100 canbe electrically coupled (“stacked”) in series to achieve higher voltageor in parallel in order to achieve higher current.

In some aspects, preferred composite membranes will include a stablecarbon backbone formed from the PVA. The PVA will be at leastsemi-crystalline, as exemplified in the general molecular schematic inFIG. 2, to afford stability in water without the need for covalentcrosslinking. In that Figure, hydrogel material 200 includes a PVAbackbone 210 with selective crosslinks 220 formed between each backbonestructure 210. The composite will also be non-ionomeric, including theabsence of any ionic groups such as sulfonate or carboxylates that mightotherwise chelate multi-valent ions, and especially ferrous and ferricions.

Examples PVA and PVA Composite Separator Preparation

Solutions of 2 wt. % and 3.5 wt. % poly(vinyl alcohol) or PVA(Sigma-Aldrich, 99+% hydrolyzed, avg. molecular weight about 130 kD) inwater were prepared by dissolution of the PVA at about 85° C. withoccasional stirring followed by cooling to room temperature, resultingin transparent, mildly viscous solutions. Microporous polymericseparators, as indicated below, were coated with the solution ofpoly(vinyl alcohol) using a doctor blade and/or by dip-coating. The wetcomposite film was dried by heating in an oven in air at 50° C. for 15minutes. The resulting composite was then soaked in pure methanol for 16hours to extract any remaining water from the composite membrane and toattempt to increase the partial crystallinity, and hence mechanicalstrength and stability in water, of the poly(vinyl alcohol) via physicalcrosslinking.

PVA films were coated using a casting knife film applicator and thenpolymerized in methanol. The thickness of the PVA coating was controlledwith the vertical adjustment of the blade. After polymerization, thecomposite separator thickness was measured using a micrometer. Theproperties of the underlying microporous support materials is summarizedin Table 1. Table 2a indicates properties of PVA-only membranes.

Ionic Conductivity Studies

Electrochemical impedance spectroscopy (EIS) was implemented using aSolartron 1280B potentiostat (Ametek, UK) to obtain the ionicconductivity of coated and uncoated separators. The ionic conductivityof both the uncoated microporous separator and PVA composite separatorwas measured in 1M HCl electrolyte in an in-house designed andfabricated conductivity cell. The in-house cell consists of two CPVCflow fields and graphite current collectors 1 cm wide and 3.69 cm long(A=3.69 cm²). The gap between each current collector and the separatorwas 1 mm as defined by a Teflon spacer. Current was collected from thiscell through two brass plates that were pressed into the back of thegraphite current collectors. Grafoil (Graftech, USA) was used tominimize contact resistances between the brass plates and the graphitecurrent collectors. The electrolyte was pumped vertically though thein-house cell via a peristaltic pump (Cole-Parmer, USA) to an externalreservoir during all measurements. The conductivity values weredetermined at room temperature. The resistance measurement without themembrane sample was subtracted from the resistance measurement with themembrane sample to yield the resistance due to the membrane. This finalresistance value was used to calculate the membrane conductivity.

Hydraulic and Ferric Ion Crossover Studies

Crossover studies were conducted with both the coated and uncoatedseparators in a 6.5 cm² cell with conductive carbon felt electrodespressed against the separator on both sides. Two separate externalreservoirs were used for both studies. During ferric ion crossoverstudies, equal flow rates were employed to ensure there was no pressuredifferential across the separator that might impact the results. Onereservoir with 250 mL of concentrated electrolyte consisting of 1M FeCl₂and 1M FeCl₃. The other reservoir consisted of 250 mL of a diluteelectrolyte with 1M FeCl₃, small amount of concentrated HCl to balancethe pH with the concentrated side, and 2M NH₄Cl to match the amount ofCl⁻ with the concentrated side. Maintaining a pH balance enables ferricand ammonium ion gradients. Crossover measurements were thus performedwith a concentration difference (delta C) of 1 Mol/liter for the ferricion. An applied potential of 0.5V across the cell allowed the flux offerric ions crossing the membrane to be detected as an oxidationcurrent. The effective diffusivity of the ferric ion, D_(eff), was thencalculated as: [(current*thickness)/(nF*deltaC)].

Correspondingly, hydraulic crossover studies were conducted with thesame 6.5 cm² cell with carbon felt electrodes on both sides. Bothreservoirs were filled with 150 mL of DI water and a few drops of HCl tomaintain conductivity. The two reservoirs were placed with a 31″ heightdifferential during flow to create a pressure differential across theseparator. The change in the liquid volume in each reservoir over timewas monitored and Darcy's law was used to extract the hydraulicpermeability. The hydraulic and ferrous ion permeation values weredetermined at room temperature.

Results

This concept was tested with two commercial microporous separators,Celgard 2325, a tri-layer material consisting of two layers of porouspolypropylene sandwiching a single layer of porous polyethylenedeveloped primarily for Li-ion batteries, and Daramic 175, a singlelayer of porous polyethylene which also contains dispersed silicaparticles intended for use in lead-acid batteries. Relevant propertiesof these materials are given in Table 1 below. Measurements of thethickness of the composite separators with a thin coating of PVA showedthat in both cases the thickness was unchanged from that of the poroussupport, indicating that the PVA was taken up into the pores of thesupport and did not form a film on the surface. When a thicker coatingwas applied, the thickness of the composite was increased. Theproperties of the composite separators were then compared to those ofthe uncoated materials and the results are given in Tables 2-4 below.

Table 2a shows characteristics of pure PVA membranes. While suchmembranes are possible, they entail certain structural/physicallimitations and were found to be more difficult to work with incomparison to the composites. The ionic conductivity of the pure PVAfilms was very high (˜54% of the electrolyte conductivity). Thisdemonstrates the high proton conductivity through the film. The thickerPVA separator maintained its mechanical integrity throughout theduration of the flow tests.

From Table 2b, it should be noted that there was negligible change inionic conductivity of the coated composite Daramic compared to theuncoated. This can be attributed to non-homogenous filling of the largerpores within. However, the coated Daramic maintained its ionicconductivity without any significant increase in areal resistance.Moreover, the ionic conductivity of the monloayer Celgard 2400 increaseddramatically with coating thickness. This suggests the filling of thenano-scale pores with the PVA films to enable enhanced wettability forelectrolyte transport. It is further posited that, not only did the PVAtake up electrolyte and allow ionic conduction, but that the PVA in thepores of the support for these composites may have improved thewettability of the support, allowing for higher level of conductivitydespite taking up part of the pore volume. With a thicker coating of PVAon either separator, the conductivity of the composite material diddrop, but remained acceptable for a flow battery.

From Table 3 it can be seen that for the Celgard separators, the PVAcomposite material had a significantly lower diffusion rate for theferrous ion, indicating that although the ionic conductivity improved;the diffusion of a reactant ion was hindered as desired. However, thisresult was not observed for the coated Daramic sample, which had asimilar or somewhat higher diffusivity for ferrous ion than the uncoatedsample. This may indicate that the amount of PVA in the Daramiccomposite was not optimal which may be related to the fact that both thepore size and pore volume in the Daramic material are larger than in theCelgard separator.

From Table 4, it can be seen that the hydraulic permeability in bothcases was lowered by incorporating PVA on and into the porous supports.This again is a desirable result, although the magnitude of the decreasein the first series of samples was small. For the second set of samples,where sufficient PVA was coated on the support to yield a significantlythicker film, the hydraulic permeability was roughly one order ofmagnitude lower. This shows that with optimization of the amount of PVAin the composite membrane, it should be possible to significantly lowerthe hydraulic permeability while maintaining an acceptable level ofconductivity.

TABLE 1a Properties of the microporous supports Thickness (μm) PorosityPore size (μm) Celgard 2325 25 39% 0.028 avg. for the polypropylenelayers Celgard 2400 25 41% 0.035 avg. for the (Polypropylene)polypropylene layers Daramic 175 175 58 ± 8% 0.1 avg. 1.0 max

TABLE 2a Ionic Conductivity of pure PVA separators in contact withLiquid Electrolyte (1M HCl). Thickness of Conductivity Areal ResistanceMembrane separator (cm) S/cm Ohm cm² “Thin” PVA 0.002-0.0025 0.116 0.022“Thick” PVA  0.01-0.0125 0.116 0.029

TABLE 2b Ionic Conductivity of PVA coated and uncoated Celgard andDaramic separators in contact with Liquid Electrolyte (1M HCl).Thickness of Conductivity Areal Resistance Membrane separator (cm) S/cmOhm cm² Uncoated Daramic 0.0175 0.11 0.16 PVA coated 0.0185 0.107 0.17Daramic −10 μm coating PVA coated 0.0195 0.106 0.18 Daramic −20 μmcoating PVA coated 0.0235 0.087 0.27 Daramic −60 μm coating UncoatedCelgard 0.0025 0.066 0.040 2400 (monolayer) PVA coated Celgard 0.00350.098 0.035 2400 −10 μm coating PVA coated Celgard 0.0045 0.110 0.0412400 −20 μm coating NAFION ® 212 0.005 0.08 0.063 (H⁺ form) NAFION ® 2120.005 0.02 0.25 (K⁺ form) NAFION ® 212 0.005 0.005 1.0 (Fe⁺² form)

TABLE 3 Ferrous Iron Effective Diffusivity of PVA coated and uncoatedCelgard and Daramic separators Thickness of Effective separatorDiffusivity Membrane (cm) cm²/s Uncoated Daramic 0.0175  0.86 − 1.6 ×10⁻⁶ PVA coated 0.0185  1.4 − 1.6 × 10⁻⁶ Daramic −10 μm coating PVAcoated 0.0218 n.d. Daramic −20 μm coating Uncoated Celgard 0.0025 0.25 −0.26 × 10⁻⁶ PVA coated Celgard 0.0035 0.15 − 0.16 × 10⁻⁶ 2400 −10 μmcoating PVA coated Celgard 0.0042 n.d. 2400 −20 μm coating NAFION ® 0.02    0.27 × 10⁻⁶ n.d. = not determined

TABLE 4 Hydraulic Permeability of PVA coated and uncoated Celgard andDaramic separators Thickness of separator k_(w)/thickness, Membrane (cm)Permeability, k_(w) cm² cm Uncoated Daramic 0.0175 1.2 × 10⁻¹³ − 1.3 ×10⁻¹³  7 × 10⁻¹² PVA coated 0.0185 8.8 × 10⁻¹⁴ − 1.3 × 10⁻¹³ 5.4 × 10⁻¹²Daramic −10 μm coating PVA coated 0.0218 1.5 × 10⁻¹⁴ − 3.5 × 10⁻¹⁴ 1.1 ×10⁻¹² Daramic −20 μm coating Uncoated Celgard 0.0025 3.0 × 10⁻¹⁴ − 3.8 ×10⁻¹⁴ 1.4 × 10⁻¹¹ PVA coated 0.0035 2.8 × 10⁻¹⁴ − 3.3 × 10⁻¹⁴ 8.6 ×10⁻¹² Celgard 2400 −10 μm coating PVA coated 0.0042 3.1 × 10⁻¹⁵ − 7.1 ×10⁻¹⁵ 1.2 × 10⁻¹² Celgard 2400 −20 μm coating NAFION ® 0.005 ≈5 × 10⁻¹⁶ 1 × 10⁻¹³

TABLE 5 Battery performance of iron flow battery over 2 days at 50° C.with PVA coated Daramic at 10 micrometers and overnight equilibration in3M HCl. Fe⁺³ crossover Voltaic (VE) and Separator current CoulombicResistance density Efficiencies (CE) Ohm cm² mA/cm² @ 100 mA/cm²Uncoated Daramic 0.54 8.1 VE 71% CE 75-90%   Coated, 1^(st) day 0.60 4.0VE 76% CE 97% Coated 2^(nd) day, 0.60 3.3 VE 73% Lower Fe conc. CE 95%

The foregoing description and examples identify various non-limitingembodiments of the invention. Use of the term “flow battery” may alsoinclude hybrid flow batteries where appropriate. References to positiveand negative electrodes should be understood in the context of adischarge reaction unless otherwise noted or apparent from the context.

Any explicit or implicit range of values indicated in this specificationnecessarily includes the disclosure of a plurality of intervals betweenthe lower and upper boundaries of the range. As a non-limiting example,a stated range of 0.01 micrometers to 1.0 micrometers encompasses anycombination of intervals between those upper boundaries, including anytwo selected from 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 evenif the specific intervals are not expressly identified. In the samemanner, any values provided in the examples will be deemed to constitutedisclosed lower or upper limits when considered in combination withother values from that same dataset. Ratios and other limitations mayalso be inferred by comparing comparable values disclosed herein.

All measurements should be read in context and with reference to themost common units and means of measurement, as will be readilyunderstood by persons of skill in this field. Unless noted to thecontrary, all measurements are conducted in ambient temperatures andpressures, with percentages referring to mass unless otherwise noted.Modifications may occur to those skilled in the art and to those who maymake and use the invention. The disclosed embodiments are merely forillustrative purposes and not intended to limit the scope of theinvention or the subject matter set forth in the claims.

What is claimed is:
 1. A flow battery system comprising: a positiveelectrolyte flowing through a positive reaction chamber and a negativeelectrolyte flowing through a negative reaction chamber, wherein atleast one of the positive electrolyte and the negative electrolyteconsists of an aqueous solution; a non-ionomeric membrane comprisinghydrogel impregnated into pores of a microporous polymeric substrate,said non-ionomeric membrane: (i) physically separating the positivereaction chamber from the negative reaction chamber so as to prevent anyflow of fluid through the pores; (ii) being ionically conductive butresistant to hydraulic crossover; and (iii) not containing any sulfonateor carboxylate ionic groups; and wherein the hydrogel consists ofpoly(vinyl alcohol).
 2. The flow battery according to claim 1, whereinthe poly(vinyl alcohol) is semi-crystalline and at least 95 wt. %hydrolyzed.
 3. The flow battery according to claim 1, wherein at leastone of the positive and negative electrolytes comprise a multi-valentionic species.
 4. The flow battery according to claim 1, wherein thehydrogel effectively blocks all pores of the microporous polymericsubstrate.
 5. The flow battery according to claim 1, wherein themicroporous polymeric substrate consists of polyethylene, polypropylene,or combinations thereof.
 6. The flow battery according to claim 1,wherein the polymeric substrate has a void volume between 30% and 80%.7. The flow battery according to claim 1, wherein the polymericsubstrate has a thickness of between 25 and 200 micrometers prior tobeing impregnated by the hydrogel.
 8. The flow battery according toclaim 1, wherein the aqueous solution has a pH of less than 3.0.
 9. Theflow battery according to claim 1, wherein the hydrogel is hydrolyzedand crosslinked.
 10. The flow battery according to claim 1, wherein boththe positive and the negative electrolytes comprise separate aqueoussolutions.
 11. The flow battery according to claim 1, wherein thenon-ionomeric membrane has a conductivity greater than 0.01 S/cmrelative to a 1M HCl solution.
 12. The flow battery according to claim1, wherein the non-ionomeric membrane has a normalized hydraulicpermeability of less than or equal to 9×10⁻¹² cm²/cm.
 13. The flowbattery according to claim 1, wherein the hydrogel is deposited on atleast one side of the substrate at a thickness of 0.1 to 25 micrometers.14. The flow battery according to claim 1, wherein the hydrogel ishydrolyzed and has a purity of at least 99 wt. %.
 15. The flow batteryaccording to claim 1, wherein the positive and negative electrolyteseach comprise a multi-valent ionic species selected from ferric orferrous ions.